Another major type of information used when creating a user is the user ID (UID). For your server, this is the only way to identify a user; usernames are just a convenience for humans who in general can't remember numbers too well. Basically, all users need a unique UID. SUSE Linux Enterprise Server 10 starts generating local UIDs at 500, and 16 bits are available for creating UIDs. This means the highest available UID is 65535, and this is also the maximum amount of local users supported on your server. Typically, UIDs less than 500 are reserved for system accounts, which are needed to start services. The UID 0 is also a special one; the user who has it has complete administrative permissions to the server. UID 0 is typically reserved for the user root.
You might want to give the same user ID to more than one user to create a backup root user. If you want to do that with useradd, execute useradd with the options -o and -u 0. For example, to make user stacey a backup root user, execute the following:
useradd -o -u 0 stacey
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